1. Field of the Invention
The invention relates to road paving equipment. More particularly, the invention relates to a method of drying roadway prior to laying asphalt and equipment for practicing the same method.
2. Description of the Prior Art
Applying asphalt pavement to roads frequently requires that the asphalt be applied in sections, because the road is too wide to process in a single pass. Thus, first one section of the road width is paved and then the second section. For reasons of simplicity, width sections will be referred to hereinafter as “lanes,” although it is understood that the actual width of the paving step does not have to correspond to a driving lane on the road. The area where one lane of asphalt overlaps another will be referred to as the “seam area.”
Typically, a first lane will be paved along a long stretch of road and the second lane paved at a later time, such as the next day or several days or even weeks later. No matter what the time difference is between paving the first lane and the second lane, the asphalt laid down on the first lane will already have cooled to what shall be referred to as “road temperature” by the time fresh hot asphalt is applied to the second lane. “Road temperature” is the temperature of the road surface, and “road surface” may include a portion of the first lane and second lane.
Asphalt is laid down warm, typically around 250 degrees F., so that it bonds with the substrate. Asphalt as it is being applied to the road surface shall be referred to hereinafter as “fresh hot asphalt,” to distinguish it from asphalt that has previously been laid down and has cooled to road temperature. After the first lane has been paved, fresh hot asphalt is applied to the second lane such that it overlaps a few inches with the asphalt of the first lane in the seam area. Ideally, the fresh hot asphalt overlapping the seam area should bond sufficiently with the asphalt of the first lane, to form a water-impervious seam. A common and well known problem, however, is that, when fresh hot asphalt is laid down on top of asphalt that is at road temperature, the difference in temperature prevents a good bond from forming. As a result, cracks form along the seam. Water then seeps through the cracks under the asphalt and down toward the sides of the roadway. Eventually, cracks form in the surface, which results further progressive deterioration of the asphalt surface.
Efforts have been made to correct this fault. Some systems have used gas torch heaters arranged so that they move ahead of the paving machinery and heat the bonding edge of the previously laid-down asphalt. One problem with gas heaters is that it is often difficult to control the heater to prevent too much heat from being applied and from being applied too directly. As a result, overheating is a common problem and, if overheated to the flash point, the asphalt can burst into flame.
An additional problem with laying asphalt is that the road surface must be very dry. If moisture has accumulated on the road surface, either from rain, dew, or melting snow or ice, or from some other event, the asphalting operation is held up until the road has dried. Most methods of drying a road surface entail blasting massive quantities of heated air onto the surface at a high velocity. Most systems using this method have no control over the temperature of the surface that is being heated, with the result that excessive heat can burn the surface and/or burn the asphalt. Such systems typically include jet engines, such as are used on racetracks, and burn at temperatures well above 1400 degrees Fahrenheit. In order to prevent damage to the surface, the engine has to be continually moving across the surface, so as not to overheat the surface. Efforts to solve this problem include limiting the air temperature to 300 degrees Fahrenheit, but that severely limits the drying capability and makes it very time-consuming to evaporate standing water from the road.
Drying a road surface with heated air is generally inefficient, especially when the jet engine is required to move at a relatively high velocity across the surface. The jet engines used to dry racetracks, for example, burn 150-200 gallons of fuel per hour. The heated air is on the surface for only a short period of time, before it moves into the surrounding air and is then wasted, with regard to heating the surface.
Typically, the efficiency of a fuel-burning heater, i.e., a burner, is limited to close to its rated capability. The ability to reduce burner output is very limited. For example, the output may be reduced to 70% of the rated capacity in some fuel-burning heaters, but any further reduction may extinguish the burner. In order to reduce the output to less than the 70%, for example, the burner may have to cycle on and off, which is an inefficient way to operate a burner. Furthermore, when the burner output is reduced, heat may be uneven, resulting in hot and cold spots.
Fuel-burning heaters are also very susceptible to wind. When using a propane heater, for example, heat is produced by burning gas being forced over a volume of air at low pressure. If there is airflow in a direction opposite the direction of flow of the burning gas, the gas will either not flow out or not burn. This results in spotty heating, with hot and cold spots.
Other efforts to dry a road surface include vacuuming the water from the surface. US Patent Application 2010/0024242 A1 discloses mobile surface drying apparatus that uses a combination blower/vacuum assembly to dry the road surface. One of the disadvantages of the apparatus is that, when using a vacuum head to suction water off the road surface, the vacuum head must be placed very close to the road surface. Road surfaces are typically uneven, which means that the vacuum head has to be placed high enough above the surface to ensure clearance of the highest points, and that leads to inconsistent results. Some areas are free of water, others not, because the vacuum head is positioned too far away from low points in the surface.
What is needed, therefore, is a method of and system for drying a road surface. What is further needed is such a method that is energy efficient and effective in removing moisture from the surface and from the surrounding air.